1. Technical Field
This invention relates generally to photoconductors for electrophotography. The invention is a positive charging, organic photoconductor material with good speed and improved stability for liquid toner electrophotography. The improved stability is a result of a positive charge injection barrier layer on top of the organic photoconductor material.
2. Related Art
In electrophotography, a latent image is created on the surface of photoconducting material by selectively exposing areas of the charged surface to light. A difference in electrostatic charge density is created between the areas on the surface exposed and unexposed to light. The visible image is developed by electrostatic toners containing pigment components and thermoplastic components. The toners are selectively attracted to the photoconductor surface either exposed or unexposed to light, depending on the relative electrostatic charges of the photoconductor surface, development electrode and the toner. The photoconductor may be either positively or negatively charged, and the toner system similarly may contain negatively or positively charged particles. For laser printers, the preferred embodiment is that the photoconductor and toner have the same polarity, but different levels of charge.
A sheet of paper or intermediate transfer medium is then given an electrostatic charge opposite that of the toner and passed close to the photoconductor surface, pulling the toner from the photoconductor surface onto the paper or intermediate medium, still in the pattern of the image developed from the photoconductor surface. A set of fuser rollers fixes the toner to the paper, subsequent to direct transfer, or indirect transfer when using an intermediate transfer medium, producing the printed image.
The important photoconductor surface, therefore, has been the subject of much research and development in the electrophotography art. A large number of photoconductor materials have been disclosed as being suitable for the electrophotographic photoconductor surface. For example, inorganic compounds such as amorphous silicon (Si), arsenic selenite (As.sub.2 Se.sub.3), cadmium sulfide (CdS), selenium (Se), titanium oxide (TiO.sub.2) and zinc oxide (ZnO) function as photoconductors. However, these inorganic materials do not satisfy modern requirements in the electrophotography art of low production costs, high-speed response to laser diode or other light-emitting-diode (LED), and safety from non-toxicity.
Therefore, recent progress in the electrophotography art with the photoconductor surface has been made with organic materials as organic photoconductors (OPC's). Typically, the OPC's in the current market are of the dual-layer, negative-charging type with a thin charge generation material layer, usually less than about 1 micron (.mu.m) thick, beneath a thicker charge transport material layer deposited on top of the charge generation layer. However, positive charging OPC's ((+)OPC's) are preferred for a discharged area developed (DAD) image as in laser printers.
Specific morphologies of phthalocyanine pigment (Pc) powder have been known to exhibit excellent photoconductivity. These phthalocyanine pigments have been used as a mixture in polymeric binder matrices in electrophotographic photoconductors, deposited on a conductive substrate.
The photoconductivity of the phthalocyanine pigment may be used to formulate the (+)OPC. Currently, known (+)OPC's may be classified as follows:
1. Single layer (+)OPC--Type I (see FIG. 1 ). The Pc is uniformly distributed throughout a relatively thick binder layer on a conductive substrate. Photons striking the upper surface of the layer generate positive and negative charges there. The generated negative charges neutralize positive charges established on the surface of the layer by the biasing corotron, discharging them. The generated positive charges travel through the bulk of the layer towards negative charges established by the biasing corotron at the conductive substrate.
In these Type I single-layer photoconductors, then, there is no need to add charge transport molecules, nor to have a separate charge transport layer. The phthalocyanine pigment content may be in the range of about 5-30 wt. %, high enough to perform both charge generation and charge transport functions, with the binder content being in the range of about 95-70 wt. %.
2. Single layer (+)OPC with charge transport molecule--TYPE II (see FIG. 2). Again, Pc in this OPC is uniformly distributed throughout a relatively thick binder layer on a conductive substrate. In addition, a charge transport molecule, called a sensitizer molecule, is also uniformly distributed throughout the binder layer. One example of a charge transport molecule is any one of the aryl-amine group of compounds. In this OPC photons tend to penetrate more deeply into the binder layer, generating positive and negative charges there. The charge transport molecule assists in the movement of these generated charges towards their respective biases.
3. Multi layer (+)OPC with charge generation layer as the top layer--TYPE III (see FIG. 3). In this OPC there is a relatively thin top layer, called the charge generation layer (CGL), on top of a relatively thick layer called the charge transport layer (CTL). The CGL contains Pc pigment uniformly distributed throughout a binder. The CTL contains a hole transport molecule, also uniformly distributed throughout a binder.
In the TYPE III OPC, photons strike the upper surface of the thinner, top layer (CGL), generating positive and negative charges there. The generated negative charges neutralize positive charges established on the surface of the CGL, discharging them. The generated positive charges travel through the CGL, and through the thicker, bottom layer (CTL) towards negative charges established at the conductive substrate.
4. Multi layer (+)OPC with charge generation layer containing charge transport molecule as the top layer--TYPE IV (see FIG. 4). This OPC is constructed in the same way as the TYPE III OPC described above, except in the upper CGL there is an additional charge transport molecule, besides the Pc, also uniformly distributed throughout the binder.
5. Multi layer (+)OPC with charge generation layer as the bottom layer--TYPE V (see FIG. 5). This OPC is constructed in the same way as the TYPE III OPC described above, except the relative positions of the CGL and the CTL are reversed--in this OPC the thinner CGL is on the bottom, and the thicker CTL is on the top.
Other layers may be added to the OPC. To improve the transfer efficiency of the toner, for example, the top surface of the OPC may be overcoated with a low surface adhesion material. This type of overcoat layer is known as a release layer. See, for example, U.S. Pat. No. 4,923,775.
The charging characteristics of the photoconductor is the most important factor for high image quality in the conventional xerographic copiers or printers. Unfortunately, the charging characteristics of the photoconductor may be easily affected by electrical or chemical contamination, and/or by physical damage to the surface incurred during the printing process. The deterioration of the charging characteristics, thus, is frequently the cause of poor print quality. Many commercially available photoconductors experience deterioration of surface charging due to the effect of mechanical wear. However, the most common cause of charge instability in the positive charging photoconductor is not only mechanical wear or damage. Instead, the instability of the surface charge is exhibited as a decrease in charge acceptance along with an increase in dark decay electrical properties of the photoconductor after repeated cycles. Charge instability is also increased at operating temperatures above room temperature.
The mechanism of the charge instability in the (+)OPC, so far, is not well understood. It is expected that the surface of the (+)OPC is more chemically vulnerable to the operating conditions such as corona charging, ozone attack, humidity, heat, etc. Especially, this phenomenon is more prominent for the (+) OPC's classified as Types I, II, III and IV above mentioned. In these (+) OPC's configurations, the hole transport components such as pigment or hole transport molecules are directly exposed to the Corona during charging. It is suspected that these (+) OPC's (Types I, II, III and IV but not V) above are more likely to exhibit deteriorated charge characteristics due to surface charge injection into the bulk of the (+)OPC. This phenomenon is more critical in (+)OPC's than in some well known inorganic photoconductors, such as amorphous selenium, CdS; etc.
Therefore, the main object of this invention is to provide a charge injection barrier for the (+) OPC which exhibits stable electrical properties, including charge acceptance, dark decay and photodischarge, in a high cycle, high severity electrophotographic process. It is known to provide a charge injection preventing layer for (+) OPC's, such as a layer of SiO2 (silica) embedded in a polymer matrix. With such kind of heterogeneous phase, however it was found that it scatters the light from the exposure source and reduces the writing incident energy. Furthermore, a severe ghosting phenomenon is frequently observed using such kinds of heterogeneous barrier materials. The ghosting image phenomenon is associated with the light fatigue effect of the photoconductive device. This phenomenon generates the residual image from the previous imaging cycle into the new print. So, it is another objective of this invention to provide a charge injection preventing layer which does not cause the ghosting and the reduced contrast image.
Presently, the (+) OPC with the added release layer discussed above to enhance toner transfer efficiency is used only in single run applications. The incorporation of the release layer on the outer layer of the OPC does not appear to contribute to surface charge stability. In some cases, it is noticed that the release layer even adversely affects the OPC's charge stability. This adverse affect is believed to be the result of leakage of the catalyst used to cross-link the release layer into the bulk of the OPC. (See U.S. Pat. No. 4,923,775.)
Another goal of the present invention is to provide the solution of the organic coating barrier for the crosslinkable top coat including poly siloxanes and the other type of the crosslinking binders. In this case, the organic coating barrier is expected to stop the photoconductor poisoning from the leaking of the catalyst or the chemicals from the top coating of polysiloxanes.
Thus, the barrier layer for the surface of the (+)OPC in the present invention is basically comprised of selected molecules or moieties which are capable of prohibiting the injection of the unwanted positive charge from the surface of the photoconductor into the bulk of the photoconductor without stopping the migration of the negative charge from the photoconductor bulk toward the surface. Such kinds of highly functional chemical species must be embedded uniformly in a selected crosslinkable polymer matrix. The selected materials and process must not cause any optical perturbance to the photoresponse process of the photoconductor, and must be robust enough in the operating environment to withstand high humidity and high temperature.